Clean up Box.move function a bit.
Clean up Box.move function a bit.

Passing in an options object will make us less likely
to make mistakes when passing in parameters and 'toMove'
as a variable name for the amount to move the box is a
more descriptive name then 'val'.

mozilla/vtt.js,mozilla/vtt.js,gkatsev/vtt.js,gkatsev/vtt.js